(1) Field of the Invention
The present invention relates to a solid-state image sensor in which a plurality of pixel cells are arranged, and particularly to a solid-state image sensor and a method of manufacturing the same that improve variations in spectral sensitivity of pixel cells.
(2) Description of the Related Art
Speaking of solid-state image sensors such as a Charge Coupled Device (CCD) sensor, shortening of exit pupil distance between a camera lens and a solid-state image element, and the miniaturization of pixel cells are progressing, along with the miniaturization of cameras and an increase in the pixel count of solid-state image sensors. Such a trend requires the establishment of technology that does not decrease light sensitivity that is the major feature of solid-state image sensors.
FIG. 1 is a cross-sectional view showing two pixel cells of a conventional solid-state image sensor. Photodiodes are two-dimensionally arranged on a semiconductor substrate. A flattening film 2 such as one made of Boron Phosphorus Silicon Glass (BPSG) is formed on the photodiodes 1, and color filters 3 are formed on the flattening film 2. Each of the color filters 3 can be a color filter of a complementary color such as yellow, cyan, magenta, and green, or a color filter of a primary color such as red, green and blue.
Color filter layers have been conventionally formed based on, for example, a dyeing method or a color resist method. In the color resist method, selective exposure and development processing are performed on a photoresist film that contains a colorant or a pigment, so as to form a color filter of intended color. Then, a transparent flattening film 4 is formed below microlenses to reduce the unevenness in the top surface of the color filters after their formation, which results in fewer variations in the form of the microlenses 5.
However, along with the shortening of exit pupil distance between a camera lens and a solid-state image element as well as the reduction in the size of pixels, it has been difficult, with the conventional solid-state image sensor, to reduce the degradation of light sensitivity and mixture of colors of adjacent pixels, by simply using thinner color filters.
According to an improved technology disclosed in the Japanese Laid-Open Application No. 05-206429 (hereinafter referred to as Patent Document 1), it is essential to reduce the distance between the top surface of the semiconductor substrate and the transparent flattening film that is located below the microlenses. To this end, the technology disclosed in Patent Document 1 proposes the use of colored microlenses 6, as shown in FIG. 2, that are capable of functioning also as color filters.
The color filter manufacturing method disclosed in Patent Document 1 makes thinner the film between the top surface of the semiconductor substrate and the flattening film located below the microlenses through the following steps: forming color filters through application, exposure, and development of a color filter resist; depositing a lens formation layer on the color filters; patterning the lens formation layer; forming the patterned lens formation layers into hemispherical shape by performing thermal flow processing or the like; and performing isotropic etching of the hemispherical-shaped resultant.
As described above, there has been proposed the use of colored microlenses so as to have them function as color filters.